Method for the accelerated dialytic recovery of foreign substances from passive collectors consisting of semipermeable membranes having different dimensions

ABSTRACT

The invention relates to a dialysis method of recovering foreign substances, particularly low-molecular weight inorganic and/or organic substances, from semipermeable membrane of varying length and dimensions, using a PLE (pressurized liquid extraction) apparatus. The membranes are preferably in the form of a tube, for example semipermeable membrane devices (SPMDs) which, in the form of passive samplers, accumulate foreign substances/pollutants on an absorbent or adsorbent.

The invention relates to a dialysis method of recovering foreignsubstances, particularly low-molecular weight inorganic and/or organicsubstances, from passive samplers, using pressurized liquid extraction(PLE). The passive samplers are comprised of semipermeable membranes ofvarying length and dimensions. More specifically, the dialysis method isused in the trace-analytical detection of semivolatile to low volatileorganic pollutants, e.g. chloroorganic pesticides, polychlorinatedbiphenyls and other chloroorganics, as well as polycyclic aromatichydrocarbons (PAH). The membranes are preferably semipermeable membranedevices (SPMDs) which, in the form of passive samplers, accumulateforeign substances/pollutants on an absorbent or adsorbent.Alternatively, the pollutants to be dialyzed may also come fromsediment, soil, old waste, vegetable or other matrices which are placedin semipermeable membranes in order to remove higher-molecular weightmatrix components from a pollutant extract, in which case noabsorbent/adsorbent is required. The dialysis method using a PLEapparatus is highly efficient, substantially speeding up the dialysis ofpollutants from semipermeable membranes.

SPMDs are preferably used in environmental monitoring as an innovativepassive sampler technology for detection and assessment of traceconcentrations of semivolatile organic pollutants (SVOCs). Said passivesamplers represent a useful alternative to the well-known, but complexactive samplers. They allow the determination of time-weighted average(TWA) concentrations over substantial periods of time. Further, the costof such sampling equipment is substantially lower compared to activesamplers, and no power supply is required. Passive samplers are used ona larger scale in monitoring the work-place atmosphere, where the issuepreferably is detecting volatile organic pollutants in the ambient air.However, the use of passive samplers in outdoor air monitoring isrelatively new. Recently, Brown R. H. [(2000): Monitoring the AmbientEnvironment with Diffusive Samplers: Theory and PracticalConsiderations. Journal of Environmental Monitoring 2, 1-9] haspublished a first review on the potential of diffusive samplers inmonitoring the outdoor air quality.

It was only in recent years that publications on the subject ofutilizing the passive sampling technology in monitoring semivolatileorganic pollutants in ambient air appeared to a minor extent. Testedpassive samplers include wax-coated paper filters, fat-coated glassplates or glass fiber filters, glass plates and textiles coated withstationary GC phase. In general, passive samplers having a highsurface-to-volume ratio, referred to as “badge type” in the Englishliterature, have been constructed in order to accumulate a sufficientquantity of pollutants from the atmosphere within a practicable periodof measurement. Such samplers represent a potential instrument toaccumulate gaseous pollutants from the air and estimate theirconcentration in the air. In “badge type” configurations, however, thereis a potential risk that the accumulation kinetics would depend on thewind speed if the sampling medium is in direct contact with theatmosphere.

In environmental monitoring the SPMDs are among the integrative passivesamplers of atmospheric and waterborne pollutants. Commerciallyavailable SPMDs consist of a tubular polyethylene membrane of smallthickness (75-90 μm), which includes a thin film of the neutralsynthetic lipophilic fatty acid ester triolein (high molecular weightof >600 Da) in the interior thereof. Lipophilic chemical substances,e.g. the chloroorganic compounds HCH isomers, polychlorinated biphenyls,chlorobenzenes, p,p′-DDT and its degradation products p,p′-DDE andp,p′-DDD, as well as PAHs, are capable of diffusing through saidsemipermeable membrane, being accumulated or concentrated there in alipid fatty acid ester, i.e., the triglyceride triolein. This ensuresdetection of environmentally relevant pollutant analytes on a tracelevel. The SPMD technology (U.S. Pat. No. 5,098,573) thus allowsdetermination of time-averaged concentrations of persistent organiccompounds dissolved in bodies of water and transported in the atmospherein the form of gases.

Initially, the method introduced by Huckins et al. [Huckins, J. N.;Manuweera, G. K.; Petty, J. D.; Mackay, D.; Lebo, J. A. (1993):Lipid-containing Semipermeable Membrane Devices for Monitoring OrganicContaminants in Water. Environ. Sci. Technol. 27, 2489-2496.] has beendeveloped to detect pollution of aquatic systems by lipophilic organicpollutants. It has later been demonstrated that SPMDs can also be usedin air monitoring of gaseous residues such as PCBs, chloroorganicpesticides and polycyclic aromatic hydrocarbons.

The use of said SPMD technology involves the disadvantage oftime-consuming sample treatment (about 48 hours) and comparatively highconsumption of solvents (520 ml) per standard SPMD tube (length: 91 cm)in dialytic recovery of organic substances accumulated on triolein (U.S.Pat. No. 5,395,426). To date, according to the state of science, eachSPMD tube is initially dialyzed for 24 hours in 260 ml of n-hexane assolvent. Thereafter, the solvent is replaced by fresh solvent, and thesample is dialyzed for another 24 hours [Huckins, J. N.; Tubergen, M.W.; Lebo, J. A.; Gale, R. W.; Schwartz, T. R. (1990). A New Approach forthe Cleanup of Organic Contaminants: Polymeric Film Dialysis in OrganicSolvent Media. J. Assoc. Off. Anal. Chem. 73, 290-293.]. However, theconcentrated final extract still includes about 4-5% of co-extractedtriolein. Subsequent purification of the extract is effected usingsilica chromatography, followed by exclusion chromatography to removetriolein residues interfering with the analysis. In addition to blockinga major working surface in the laboratory, simultaneous processing of anumber of samples implies a high input of time and labor. Moreover, thecomparatively high laboratory price for a standard SPMD 91 cm in lengthfrom EST, Inc., USA (Environmental Sampling Technologies), the onlyproducer at present, being about 50 to 60 ε apiece, represents anobstacle to systematic investigations, because substantial numbers ofunits would be required to this end. The use of only 10 such SPMDs wouldimply a financial expenditure as high as about 500-600 ε.

The invention was therefore based on the object of developing alow-cost, efficient method for routine use, which method would simplifyand significantly shorten said dialytic recovery of foreign substances,e.g. low-molecular weight inorganic and/or organic pollutants inenvironmental monitoring, from semipermeable membranes.

The invention is carried out in accordance with the claims.Surprisingly, foreign substances or pollutants can be recovered bydialysis much more rapidly from semipermeable membranes of varyinglength and dimensions when using a PLE (pressurized liquid extraction)apparatus. To this end, the foreign substance-loaded semipermeablemembrane to be dialyzed is fixed in a netlike mask of inert materials inan extraction cartridge of the PLE apparatus. Dialysis is performed withan organic solvent or mixture of solvents, optimizing the operatingvariables solvent, pressure, temperature, and number of time-dependentextraction steps of the PLE apparatus.

By virtue of the present invention, a modern extraction device forperforming dialyses, specifically constructed for extracting pollutantsfrom environmental matrices and other matrices, is provided for thefirst time. To avoid adhering of the tube to the inner wall of thecartridge, which would hamper efficient dialysis, the semipermeablemembrane is incorporated in the netlike mask of inert materials in theextraction cartridge.

In principle, so-called semipermeable passive samplers in the meaning ofthe invention are all those systems known from the prior art whichconsist of semipermeable membranes of varying length and dimensions. Inone preferred embodiment they assume the form of a tube. In principle,however, other forms such as bag-shaped or pouch-shaped embodiments canalso be used. Semipermeable passive samplers are also understood to bematerials which include foreign substances or pollutants for the purposeof purifying a pollutant extract from higher-molecular weight matrixcomponents and are embedded in semipermeable membranes of varying lengthand dimensions.

In addition to improved methodical standardization, the advantages ofthis method can be seen in an enormous reduction in time of previousconventional dialysis methods from 48 hours down to a static time ofdialysis of <2 hours, and as a rule, preferably about 40 minutes. Thecapability of using semipermeable membranes of varying length anddimensions leads to a significant decrease in the consumption of solventand to savings of material cost, because SPMDs shorter than thosecommercially available at present, with a length of 91 cm, can also beused and dialyzed efficiently.

The retrieval rates of the method according to the invention correspondto those achieved by means of the conventional dialysis methodpreviously used. In fact, a significant increase of the retrieval ratefrom 80 to 90% was achieved in the HCH group in particular. Suchimprovements permit routine use of the method according to theinvention.

The method of pressurized liquid extraction (PLE) is per se known. Toperform said method, various apparatus are available on the market, bymeans of which the so-called accelerated solvent extraction can beperformed to extract solid and pasty samples. Thus, one well-knownmanufacturer is the American company Dionex which has the company nameDionex GmbH in Germany and, among other things, has the extractionapparatus ASE 200 and ASE 300, as well as ASE 100 (as from May 2002) onthe market (ASE®=accelerated solvent extraction). These apparatus allowthe use of solvents of most various polarities, alone or in mixture. Theextraction temperatures can be adjusted in a range of from roomtemperature up to 200° C. at pressures of between 3.5 (required initialpressure to operate the apparatus) and 20 MPa, the pressure merelyhaving the function of maintaining the employed solvent in liquid stateduring the extraction process. A sample carousel offers the opportunityof performing 24 previously prepared extraction processes in immediatesuccession via a PC control device. For extraction processes, theextraction apparatus ASE 200 provides stainless steel cartridges havinga capacity of 1, 5, 11, 22 and up to a maximum of 33 ml, and cartridgesup to a maximum of 100 ml in the ASE 300 and ASE 100.

According to the invention, the operating variables solvent, pressure,temperature, and number of time-dependent extraction steps wereoptimized to meet the requirements of dialysis.

Compared to the previously applied dialysis method, the method of theinvention achieves comparable recovery values within substantiallyshorter periods of dialysis, without impairing the quality of themembrane passive sampler (influence on pore size or dissolution of tubecomponents).

In a preferred embodiment of the invention the pollutant-loaded membraneis a semipermeable membrane wherein the pollutants are presentaccumulated on an absorbent or adsorbent. Preferred is an SPMD includingat least one hydrophobic absorbent or adsorbent of high accumulativecapacity for semivolatile organic pollutants. Absorbents or adsorbentshaving high accumulative capacity are well-known to those skilled in theart. It is possible to use the well-known liquid fatty acid estertriolein (in case of lipophilic pollutants), but also, solid organicabsorbents or adsorbents such as Lichrolut® (copolymer, Merck,Darmstadt, Del.), XAD® (adsorber resin, Serva, Heidelberg, Del.), Tenax®(polymer, Supelco, Taufkirchen, Del.), silicone and/or active carbon,and materials having comparable hydrophobicity can be included asabsorbents or adsorbents in a semipermeable passive sampler.

Alternatively, a membrane made of low-density polyethylene can also bean SPMD having sediment, soil, old waste, vegetable or other matricescontaminated in particular with low-molecular weight pollutants placedtherein. Such placing is preferably effected by mechanical opening ofthe membrane, filling in the material, and subsequent sealing e.g. bymeans of a thermosealed seam.

According to the invention, this variant results in an improved cleanupof polluted materials. Pure extracts free of higher-molecular weightmatrix-dependent interfering substances are obtained, which allow a moresensitive analysis than heretofore. An absorbent or adsorbent is notrequired.

In a preferred embodiment of the invention the netlike mask that is usedfor the semipermeable membrane is made of metal or ceramics, morepreferably of stainless steel. The netlike mask, having the shape andsize of the cartridge, ensures optimal and reproducible fixing of themembrane (cf., FIG. 1, SPMD tube in front of a metal mask of stainlesssteel), with no contact between the membrane and the inner wall of theextraction cartridge. FIG. 2, from the left to the right, illustrates apreferred metal net with the membrane inserted therein, the insertion ofthe metal net into the cartridge that is used, and the cartridgere-sealed with membrane and metal net for the dialysis process. Theclose-meshed stainless steel net preferably used to this end consists ofstainless steel 1 mm in thickness. Flooding of the membrane with thedialysis solvent is ensured in an optimal fashion.

For dialytic recovery, the PLE extraction equipment that is used isadjusted in its extraction parameters, i.e., pressure, temperature,solvent, as well as number and duration of the static extraction steps,in such a way that soluble higher-molecular weight substances (molecularweight >600 g/mol) included inside the membrane would not be co-dialyzedat all, or optionally to only a very small extent (for example, trioleinup to 5%). Dialysis is carried out at a pressure in the PLE apparatuswhich is higher than atmospheric pressure, and in a preferred fashionthe pressure is adjusted to about 3.5 MPa.

The temperature in the PLE apparatus is adapted to the durabilityproperties of the semipermeable membrane, and in a particularlypreferred fashion the dialysis proceeds at room temperature and up to50° C.

Preferred solvents are nonpolar organic solvents or a mixture ofwell-miscible nonpolar and polar solvents, such as n-hexane or toluene,as well as mixtures of cyclohexane and ethyl acetate, cyclohexane andacetone, n-hexane and dichloromethane, n-hexane and dichloromethane andn-hexane/and acetone. A solvent mixture of n-hexane and acetone at aratio of from 90:10 to 50:50 was found to be particularly advantageousin the dialysis of semipermeable passive samplers (see Table 1). Whilein the preferred variant of using a commercially available SPMD withtriolein it has been noted that the efficiency of dialysis decreaseswhen using a higher amount of polar acetone in the mixture (e.g.n-hexane/acetone 50:50), there is a simultaneous decrease in the lipidcontent of triolein from 4.5% (10% acetone) via 2.5% (30%) and down to0.8% (50%) when increasing the amount of acetone in the dialysisextract. TABLE 1 Solvent dependence of recovery values (all figures in%) n-hexane/ acetone n-hexane/acetone n-hexane/acetone substancen-hexane 90:10 70:30 50:50 α-HCH 74 89 88 58 β-HCH 88 90 81 87 γ-HCH 81119 114 75 δ-HCH 87 90 81 85 HCBz 88 107 110 62 PCB 28 82 113 113 84 PCB52 135 105 101 79 PCB 101 101 99 95 85 PCB 138 107 98 94 91 PCB 153 10698 94 90 PCB 180 120 99 91 91 Phen 98 130 130 78 Ant 93 110 110 68 Fluor96 114 115 91 Pyr 93 114 110 90 BaP 76 99 84 85

In case of deficiencies in the analysis when determining analytes at alipid content of about 4.5% (n-hexane/acetone 90:10), the use of the70:30 mixture with a lipid content of only 2.5% is preferred.

The time of dialysis of the static dialysis steps is substantially below24 hours, preferably below 2 hours, but is normally no longer than 4×10minutes.

In a particularly preferred embodiment of the invention commerciallyavailable SPMDs are dialyzed at a pressure of 3.5 MPa and at atemperature of 50° C. for 4×10 minutes, using a solvent mixturecomprised of n-hexane and acetone (90:10 v/v).

The method represents an efficient and optimal solution for the dialyticrecovery of pollutants from most various matrices.

The method utilizes diverse possible variations of the extractionparameters of a pressurized liquid extraction (PLE), specifically indialytic recovery of pollutants from integrative semipermeable passivesamplers, preferably from SPMDs.

The method of the invention has many advantages:

The previous dialysis method is simplified, made significantly shorterwith regard to time, promoting routine use of e.g. passive samplers,preferably SPMDs, in the domain relating to the terrestrial and aquaticenvironment, or in work-place monitoring.

In summary, it can be stated that the invention represents a so-called“accelerated dialysis procedure” (ADP) using a pressurized liquidextraction (PLE) apparatus, wherein the variable operating parameterspressure, temperature, solvent, time of dialysis, and number of dialysissteps have been optimized. The method is preferably used to detect tracequantities of airborne organic pollutants, such as the nonpolarchloroorganic compounds HCH isomers, DDT and metabolites thereof, PCBsand PAHs, in semipermeable passive samplers.

In the dialysis of commercially available passive samplers the inventionpresented herein results in a considerable decrease in the time ofdialysis (<2 hours, preferably 4×10 minutes instead of 48 hours), withsignificantly lower consumption of solvent (120 ml instead of 520 ml).This can be achieved by incorporating an inert net to fix andmechanically stabilize the semipermeable membrane in the extractioncartridge, and in this way there is optimum flooding of the membrane bythe solvent during the process of dialysis in simultaneous combinationwith dialysis-related optimization of the operating variables. Also,merely by varying the polarity of the solvent as a result of thepreferred use of said n-hexane/acetone mixture, a significantimprovement of the recovery values for analytes included in the group ofHCH isomers is possible without deteriorating the corresponding recoveryvalues for analytes of other groups of pollutants.

By virtue of the present invention, the use of a modern extractionapparatus for performing dialyses, specifically constructed forextracting pollutants from environmental matrices and other materials,is possible for the first time.

In addition to improved methodical standardization, the advantages ofthis method can be seen in an enormous reduction in time compared toprevious conventional dialysis methods, a significant decrease in theconsumption of solvent, and in saving of material cost. The retrievalrates correspond to those achieved by means of the conventional dialysismethod previously used. In fact, a significant increase of the retrievalrate from 80 to 90% was achieved in the HCH group in particular. Suchimprovements permit routine use of this method.

Moreover, PLE apparatus in combination with the dialysis methodaccording to the invention can also be used for accelerated dialysis inother fields of application using semipermeable membranes, e.g. in thedialysis of foreign substances from medical, biochemical, biological,and technical processes.

EXAMPLE

The dialysis method was performed under the following conditions in anASE 200 extraction apparatus including an SPMD 10 cm in length:

-   -   Insertion of the SPMD situated in a metal net of stainless steel        into a 33 ml extraction cartridge    -   Adjustment of optimum pressure: 3.5 MPa    -   Adjustment of optimum temperature with no damaging effect on the        SPMD material: 50° C.    -   Dialysis solvent mixture with optimum effect: n-hexane/acetone        90:10 at a maximum lipid content of 4.5% (triolein)    -   Optimum time of dialysis for high recovery values: 4×10 minutes,        using 30 ml of fresh solvent each time

The results of quintuple determinations under such optimized conditionsare listed in Table 2. The mean values of HCHs were in the range of from88% for δ-HCH and up to 100% for γ-HCH, 106% for hexachlorobenzene,about 100% for PCBs and PAHs. The RSD values, ranging between 5.1 and19.0%, are deemed acceptable, particularly since the tests wereperformed at substantial time intervals, using spiked pollutantconcentrations. TABLE 2 Recovery values of optimized accelerateddialysis (solvent: n-hexane/ acetone 90:10, pressure: 3.5 MPa,temperature: 50° C.) recovery [%] mean RSD substance 1 2 3 4 5 [%] [%]α-HCH 85 90 96 89 103 93 10.8 β-HCH 83 95 96 90 95 92 12.0 γ-HCH 85 9497 119 103 100 19.0 δ-HCH 77 91 80 90 102 88 13.6 HCBz 109 120 106 10789 106 16.0 PCB 28 112 122 110 113 87 109 11.9 PCB 52 105 118 107 105 93106 12.3 PCB 101 104 112 95 99 111 104 8.7 PCB 138 99 110 97 98 102 1018.9 PCB 153 98 104 100 98 95 99 5.1 PCB 180 93 102 98 99 97 98 5.1 Phen123 134 121 130 105 123 14.6 Ant 100 92 87 110 95 97 13.4 Fluor 118 127116 114 100 115 13.0 Pyr 116 124 112 114 90 111 18.9 BaP 82 84 85 99 9389 11.2

1. A method for the dialytic recovery of foreign substances from passivesamplers consisting of semipermeable membranes of varying length anddimensions, characterized in that a semipermeable passive sampler loadedwith foreign substances is fixed in a netlike mask of inert materials inan extraction cartridge and dialysis is carried out with a solvent or amixture of solvents, using a pressurized liquid extraction (PLE)apparatus.
 2. The method according to claim 1, characterized in that thepassive sampler is tubular in shape and includes at least onehydrophobic absorbent or adsorbent of high accumulative capacity forsemivolatile organic pollutants.
 3. The method according to claim 2,characterized in that the absorbent/adsorbent of high accumulativecapacity is triolein, Lichrolut, XAD, Tenax, silicone, and/or activecarbon, or a material having comparable hydrophobicity.
 4. The methodaccording to claim 1, characterized in that the netlike mask consists ofmetal or ceramics.
 5. The method according to claim 1, characterized inthat the dialysis is carried out at a pressure in the PLE apparatuswhich is higher than atmospheric pressure, preferably 3.5 MPa.
 6. Themethod according to claim 1, characterized in that the temperature is60° C. at maximum, preferably room temperature (20° C.) and up to 50° C.7. The method according to claim 1, characterized in that n-hexane ortoluene, as well as mixtures of cyclohexane and ethyl acetate,cyclohexane and acetone, n-hexane and methylene chloride, n-hexane anddichloromethane, as well as n-hexane and acetone are used as solvents.8. The method according to claim 7, characterized in that a solventmixture of n-hexane and acetone at a ratio between 90:10 and up to 50:50is used in the dialysis of semivolatile to low volatile organicpollutants.
 9. The method according to claim 1, characterized in thatsemivolatile to low volatile organics are dialyzed for at least 4×10minutes.
 10. Use of the method according to claim 1 in the dialysis ofpollutants from environmental monitoring and from medical, biochemicaland biological sectors.
 11. Use of the method according to claim 1 inthe dialysis of sediment, soil, old waste, or vegetable matrices,preferably including low-molecular weight pollutants, said materialsbeing placed in the semipermeable passive samplers.